THE TYMPANUM. 



635 



US. 



the Tipper part of the pharynx. From the tympanum it is directed 

 forwards and inwards, with a slight indination downwards ; its entire 

 length is about an inch and a half. The osseous division of the 

 Eustachian tube, already described in the Osteology, is placed at the 

 angle of junction of the petrous portion of the temporal bone with the 

 squamous portion. The anterior part of the tube is formed of a tri- 

 angular piece of cartilage, the edges of which are slightly curled round 

 towards each other, leaving an interval at the under side, in which the 

 canal is completed by dense but pliable fibrous membrane. The tube is 

 trumpet-shaped, being narrow behind, and gradually expanding until 

 it becomes wide in front ; the anterior part is compressed from side to 

 side, and is fixed to the inner pterygoid jDrocess of the sphenoid bone. 

 The anterior opening is oval in form, and is placed obliquely at the side 

 and upper part of the pharynx, into which its prominent margin 

 projects behind the lower meatus of the nose, and above the level of 

 the hard palate. Through this aperture the mucous membrane of the 

 pharynx is continuous with that which lines the tympanum, and under 

 certain conditions air passes into and out of that cavity. 



SMALL BONES OF THE EAR. 



Three small bones (ossiaila avditus) are contained in the upper part of 

 the tympanum: of these, the outermost {malleus) is attached to the mem- 

 brana tympani ; the innermost 

 (stapes) is fixed in the fenestra 

 ovalis ; and the third (mcus) 

 placed between the other two, 

 is connected to both by articu- 

 lar surfaces. The malleus and 

 incus are in direction nearly 

 vertical, the stapes horizontal. 

 They form together an angular 

 and jointed connecting rod be- 

 tween the membrana tympani. 

 and the membrane which closes 

 the fenestra ovalis. 



The malleus or hammer bone 

 (fig. 448, a), consists of a central 

 thicker portion, with processes 

 of different lengths. At the 

 upper end of the bone is a 

 rounded head (capitulum) (fig. 

 448, A, 1), which presents in- 

 ternally and posteriorly an ir- 

 regularly oval surface covered 

 with cartilage, for articulation 

 with the incus. Below the head 

 is a constricted neck (cervix) ; 

 and beneath this another shght 

 enlargement of the bone, to 

 which the processes are at- 



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Fig. 448. — Boxes of the Tympanum of 

 THE Right Side (from Arnold). Twice 

 THE Natural Size. 



A, malleus ; 1, its head ; 2, the handle ; 

 3, long or slender process ; 4, short process. 

 B, incus ; 1, its body; 2, the long process with 

 the orbicular process ; 3, .short or posterior 

 process ; 4, articular surface receiving the 

 head of the malleus. C, stapes ; 1, head ; 

 2, posterior crus ; 3, anterior cms ; 4, base. 

 C*, base of the stapes. D, the three bones in 

 their natural coun(c:ion as seen from the out- 

 side ; a, malleus ; b, incus ; c, stapes. 



tached. The handle (manubrium) 

 of the malleus (fig. 448, A, 2) is 

 a tapering and sliglitJy twisted process, compressed from before back- 



